Polyfluoroethylene dispersions find wide application in the coating industry due to the unique performance of the coatings in respect of e.g. release properties, good weathering resistance, and flame retardancy. They are mainly used for coating kitchenware, chemical apparatus and glass fabrics. In many such applications, the dispersions are applied at relatively high solids contents, e.g., up to 70-75% by weight. These concentrated dispersions are mainly stabilized by nonionic emulsifiers such as alkylarylpolyethoxy alcohols and alkylpolyethoxy alcohols, using colloid-chemistry methods.
There are in principle two different polymerization processes for preparing fluoropolymers, namely suspension polymerization leading to polymer granules and, on the other hand the process known as emulsion polymerization (also called dispersion polymerization), leading to an aqueous colloidal dispersion. This invention concerns a purification process which can be notably applied to dispersions obtained from emulsion polymerization.
The manufacture of such fluoropolymer dispersions, especially tetrafluoroethylene (TFE) polymers (homopolymers and copolymers) comprises, generally, an emulsion polymerisation step.
In all of these emulsion polymerizations, a surfactant or emulsifier is required to stabilize fluoropolymer dispersion in aqueous medium; said surfactant is generally chosen as not to disrupt the polymerization by chain transfer. These surfactant are termed nontelogenic emulsifiers (U.S. Pat. No. 2,559,752). Use is mainly made of perfluorooctanoic acid (PFOA) in the form of ammonium and/or alkali metal salts, particularly preferred is anmoniun perfluorooctanoate (APFOA). The content of this surfactant is generally within the range from 0.02 to 1% by weight, based on the fluoropolymer.
Also, other fluorinated surfactants can be used. For example,    Patent Citation 0001: EP 822175 A (DU PONT DE NEMOURS). 1998 Feb. 4,describes the use of salts of CH2-containing fluorocarboxylic acids for the emulsion polymerization of TFE.    Patent Citation 0002: WO WO 97/08214 (DU PONT DE NEMOURS). 1997 Mar. 6describes the use of 2-perfluorohexylethanesulfonic acid or salts thereof for TFE polymerization.
Other (per)fluorooxyalkylenic surfactants useful for fluoropolymers emulsion polymerization have been described in following patent applications:    Patent Citation 0003: US 2007015864,    Patent Citation 0004: US 2007015865,    Patent Citation 0005: US 2007015866,    Patent Citation 0006: US 2006015937,    Patent Citation 0007: US 2007025902,    Patent Citation 0008: US 2007027251.    Patent Citation 0009: U.S. Pat. No. 2,559,752 (DU PONT DE NEMOURS). 1951 Jul. 10,describes other fluorinated surfactants, but these have not been widely used since their volatility is low. These chemicals can cause discoloration of the final products at high processing temperatures.
While fluoropolymer aqueous emulsions obtained from polymerization generally have a solids content of about 15 to 40% by weight, industrial utilization thereof requires said dispersions to be concentrated up to about 40 to 75% by weight. A first example of concentration processes is notably a decantation or phase-separation process (also known as clouding process) as described for instance in    Patent Citation 0010: U.S. Pat. No. 3,037,953 B (DU PONT DE NEMOURS). 1962 Jun. 5,    Patent Citation 0011: U.S. Pat. No. 3,704,272 B (DU PONT DE NEMOURS). 1972 Nov. 28,    Patent Citation 0012: U.S. Pat. No. 3,301,807 B (THIOKOL CHEMICAL CORP). 1967 Jan. 31, whose disclosures are incorporated herein by reference, said phase-separation process generally comprising adding a non-ionic surfactant to an aqueous fluoropolymer emulsion and heating the mixture to a temperature non lower that the cloud point of the non-ionic surfactant to thereby cause separation into an aqueous phase and a fluoropolymer phase. Another example of concentration process is an ultrafiltration process or dialysis process as described for instance in    Patent Citation 0013: U.S. Pat. No. 4,369,266 B (HOECHST AG). 1983 Jan. 18,    Patent Citation 0014: U.S. Pat. No. 6,136,893 B (DAIKIN IND LTD). 2000 Oct. 24.whose disclosures are incorporated herein by reference.
Nevertheless, in the techniques listed above for raising concentration, the majority of the fluorinated surfactants remain in the fluoropolymer dispersions.
Thus, during subsequent use of these dispersions, workers can be exposed to contact with the fluorinated surfactants, in particular the PFOA.
It is also known that in the applications of fluoropolymer dispersions, e.g. as coating or in the impregnation of glass fibres, said fluorinated surfactants can reach the environment. Fluorinated surfactants can be released in the environment notably in the washing effluent or can be released in the atmosphere during drying or sintering steps which the fluoropolymer is submitted to during processing. The latter emission is still more pronounced when coatings are produced comprising PFOA (or derivatives thereof) as emulsifier, since PFOA is highly volatile.
However, some of said fluorinated surfactants have been recently classified as hazardous to the environment; their long residence time in living animals (e.g. in human beings) has been demonstrated. Perfluorooctanoic acid (PFOA) and corresponding derivatives have been particularly investigated as contaminant; a US Environmental Protect Agency (EPA) Science Advisory Board has announced on June 2005 a preliminary finding that PFOA is a likely carcinogen; toxicological studies to finally assess effects of this contaminant on animals and human beings are still under progress.
On Jan. 25, 2006, EPA invited fluoropolymer and telomer manufacturers to participate in a global stewardship program on PFOA and related chemicals, and particularly to commit to achieve, no later than 2010, a 95% reduction, measured from a year 2000 baseline, in both facility emissions to all media of PFOA, precursor chemicals that can break down to PFOA, and related higher homologue chemicals, and product content levels of PFOA, precursor chemicals that can break down to PFOA, and related higher homologue chemicals and to commit to working toward the elimination of PFOA, PFOA precursors, and related higher homologue chemicals from emissions and products by five years thereafter, i.e. no later than 2015.
There is thus a strong environmental pressure for substantially reducing PFOA emission, and thus to develop suitable PFOA recovery processes enabling achieving these goals.
In addition, surfactants such as fluorocarboxylic acid and their salts (such as e.g. CH2-containing fluorocarboxylic acids described in,    Patent Citation 0015: EP 822175 A (DU PONT DE NEMOURS). 1998 Feb. 4) generally decompose by decarboxylation at the sintering temperatures normally employed, from 350 to 450° C., to give carbon dioxide and fluorinated hydrocarbons, which have a major global-warming effect (“greenhouse effect”).
There is thus a strong demand from transformers and final users of fluoropolymer dispersions for producers to provide for said fluoropolymer dispersions substantially free from fluorinated surfactants, in particular from perfluorooctanoic acid and derivatives thereof.
Several solutions have been proposed in the past for the purification of fluoropolymer dispersions from fluorinated surfactants, including phase separation and ion exchange treatments.
Thus,    Patent Citation 0016: EP 1489104 A (ASAHI GLASS CO LTD). 2004 Dec. 22,discloses a process for producing an aqueous polytetrafluoroethylene (PTFE) composition purified from anionic perfluorocarboxylate surfactants (APFC), said process comprising adding a nonionic surfactant and water to an aqueous PTFE emulsion, concentrating the resulting PTFE aqueous dispersion by phase separation and sedimentation and separating the highly concentrated aqueous PTFE dispersion containing a small amount of the APFC from the supernatant containing a large amount of said APFC.    Patent Citation 0017: EP 1574527 A (DAIKIN IND LTD). 2005 Sep. 14,discloses a method of purifying an aqueous fluoropolymer emulsion containing 15-35% by mass of fluoropolymer from fluorine-containing surfactants without lowering its dispersibility, said method comprising purifying an aqueous fluoropolymer emulsion by a specific technique of concentration comprising concentration by phase separation, electric concentration and/or ion exchange concentration.    Patent Citation 0018: WO WO 2005/003190 A (DAIKIN IND LTD). 2005 Jan. 13,discloses a method for removing a fluorine-containing surfactant from a material to be treated, characterized in that it comprises contacting said material with a substance [A] which is a gas under standard conditions, such as supercritical carbon dioxide, to thereby remove the fluorine-containing surfactant.
Purification methods including an ion exchange treatment are generally preferred because of their lower fluorinated surfactant final concentrations in the dispersions.    Patent Citation 0019: U.S. Pat. No. 6,833,403 B (3M INNOVATIVE PROPERTIES). 2004 Dec. 21.discloses a process for removing fluorine-containing emulsifiers from fluoropolymer dispersions, Said process comprising adding a non-ionic emulsifier to the fluoropolymer dispersion and contacting said stabilized dispersion with a basic anion exchanger. Removal of fluorinated surfactants is carried out either with crude dispersions from polymerization having a solids content of 15 to 30% by weight or with previously concentrated dispersions with a solids content of up to 70% weight.    Patent Citation 0020: U.S. Pat. No. 6,720,360 B (3M INNOVATIVE PROPERTIES). 2004 Apr. 13.discloses a process for preparing salt-free fluoropolymers, so-called ultra-clean fluoropolymers, said process comprising, inter alia, submitting fluoropolymer lattices to an ion exchange step, wherein both cation and anion exchangers treatments are carried out on raw dispersions from polymerization having a solids content from 10 to 40% by weight, said dispersions being combined with a non-ionic emulsifier to decrease solids content to less than about 20% wt.    Patent Citation 0021: U.S. Pat. No. 6,825,250 B (3M INNOVATIVE PROPERTIES). 2004 Nov. 30.discloses a process for reducing the amount of fluorinated emulsifier in an aqueous fluoropolymer dispersion by contacting said aqueous fluoropolymer dispersion comprising between 10 and 70% wt of fluoropolymer with an anion exchange resin by agitating the aqueous fluoropolymer dispersion with an effective amount of anion exchange resin and separating the anion exchange resin from the aqueous fluoropolymer dispersion.    Patent Citation 0022: EP 1533325 A (3M INNOVATIVE PROPERTIES). 2005 May 25.discloses a method of making an aqueous dispersion of non-melt processible polytetrafluoroethylene having low amount of fluorinated surfactant, the method comprising, inter alia, emulsion polymerizing tetrafluoroethylene (TFE) in the presence of free radicals capable of introducing ionic end groups or precursors thereof in the PTFE polymer to yield a dispersion having a solids content between 10 and 35% wt, and reducing the amount of fluorinated surfactant in the thus obtained aqueous dispersion, e.g. by contacting the dispersion with an anion exchanger.    Patent Citation 0023: EP 1584632 A (SOLVAY SOLEXIS S.P.A). 2005 Oct. 12.discloses a process to substantially remove the fluorinated anionic surfactants from a fluoropolymer dispersion having a solids content from 20 to 75% weight, said process comprising, inter alia, adding a salt, soluble in water and not giving precipitates in the dispersion under the use conditions and contacting the dispersion with a basic anionic exchanger.
Nevertheless, the ion exchange treatments of the processes of the prior art still suffer from several drawbacks; the efficiency of fluorinated surfactant removal is limited due to the strong affinity of said fluorinated surfactant for the fluoropolymer particles. Moreover, the risk of clogging or jamming the ion exchange resins cannot be avoided and undesired plant shut downs occur due to these phenomena. Also, a considerable amount of mechanical energy is required for circulating the aqueous dispersion through the ion exchange beds: high shear conditions can thus occur, yielding uncontrolled and undesired coagulation phenomena. Finally, agglomerates formation cannot be avoided during said ion exchange treatment: thus, purified dispersion can comprise agglomerates which might generate, if not removed with yield loss and energy consumption, undesirable defects in final parts (films, coating).